Intermittent running: muscle metabolism in the heat and effect of hypohydration

Implications of Heat Stress-induced Metabolic Alterations for Endurance Training

Inducing a heat-acclimated phenotype via repeated heat stress improves exercise capacity and reduces athletes̓ risk of hyperthermia and heat illness. Given the increased number of international sporting events hosted in countries with warmer climates, heat acclimation strategies are increasingly popular among endurance athletes to optimize performance in hot environments. At the tissue level, completing endurance exercise under heat stress may augment endurance training adaptation, including mitochondrial and cardiovascular remodeling due to increased perturbations to cellular homeostasis as a consequence of metabolic and cardiovascular load, and this may improve endurance training adaptation and subsequent performance. This review provides an up-to-date overview of the metabolic impact of heat stress during endurance exercise, including proposed underlying mechanisms of altered substrate utilization. Against this metabolic backdrop, the current literature highlighting the role of heat stress in augmenting training adaptation and subsequent endurance performance will be presented with practical implications and opportunities for future research.

Partial and Complete Fluid Replacement Maintains Exercise Performance in a Warm Environment Following Prolonged Cold-Water Immersion

ABSTRACT

Wheelock, CE, Stooks, J, Schwob, J, Hess, HW, Pryor, RR, and Hostler, D. Partial and complete fluid replacement maintains exercise performance in a warm environment following prolonged cold-water immersion. J Strength Cond Res 38(2): 290-296, 2024-Special warfare operators may be exposed to prolonged immersion before beginning a land-based mission. This immersion will result in substantial hypohydration because of diuresis. This study tested the hypothesis that both partial and full postimmersion rehydration would maintain performance during exercise in the heat. Seven men (23 ± 2 years; V̇o2max: 50.8 ± 5.3 ml·kg-1·min-1) completed a control trial (CON) without prior immersion and 3 immersion (18.0°C) trials without rehydration (NO) or with partial (HALF) or full (FULL) rehydration. After immersion, subjects completed a 60-minute weighted ruck march (20.4 kg; 5.6 kph) and a 15-minute intermittent exercise protocol (iEPT) in a warm environment (30.0°C and 50.0% relative humidity). The primary outcome was distance (km) covered during the iEPT. A priori statistical significance was set to p ≤ 0.05. Immersion resulted in 2.3 ± 0.4% loss of body mass in all immersion trials (p < 0.01). Distance covered during the first 13-minute interval run portion of iEPT was reduced in the NO rehydration trial (1.59 ± 0.18 km) compared with all other conditions (CON: 1.88 ± 0.18 km, p = 0.03; HALF: 1.80 ± 0.18 km, p < 0.01; FULL: 1.86 ± 0.28 km, p = 0.01). During the final 2 minutes of the iEPT, distance in the NO rehydration trial (0.31 ± 0.07 km) was reduced compared with the FULL rehydration trial (0.37 ± 0.07 km; p < 0.01) but not compared with CON (0.35 ± 0.07 km; p = 0.09) or HALF (0.35 ± 0.07 km; p = 0.08). Both partial and full postimmersion fluid replacement maintained intermittent exercise performance and should be applied as rehydration strategies.

Acute exercise in a hot environment increases heat shock protein 70 and peroxisome proliferator-activated receptor γ coactivator 1α mRNA in Thoroughbred horse skeletal muscle

Heat acclimatization or acclimation training in horses is practiced to reduce physiological strain and improve exercise performance in the heat, which can involve metabolic improvement in skeletal muscle. However, there is limited information concerning the acute signaling responses of equine skeletal muscle after exercise in a hot environment. The purpose of this study was to investigate the hypothesis that exercise in hot conditions induces greater changes in heat shock proteins and mitochondrial-related signaling in equine skeletal muscle compared with exercise in cool conditions. Fifteen trained Thoroughbred horses [4.6 ± 0.4 (mean ± SE) years old; 503 ± 14 kg] were assigned to perform a treadmill exercise test in cool conditions [COOL; Wet Bulb Globe Temperature (WBGT), 12.5°C; n = 8] or hot conditions (HOT; WBGT, 29.5°C; n = 7) consisting of walking at 1.7 m/s for 1 min, trotting at 4 m/s for 5 min, and cantering at 7 m/s for 2 min and at 90% of VO2max for 2 min, followed by walking at 1.7 m/s for 20 min. Heart rate during exercise and plasma lactate concentration immediately after exercise were measured. Biopsy samples were obtained from the middle gluteal muscle before and at 4 h after exercise, and relative quantitative analysis of mRNA expression using real-time RT-PCR was performed. Data were analyzed with using mixed models. There were no significant differences between the two groups in peak heart rate (COOL, 213 ± 3 bpm; HOT, 214 ± 4 bpm; p = 0.782) and plasma lactate concentration (COOL, 13.1 ± 1.4 mmoL/L; HOT, 17.5 ± 1.7 mmoL/L; p = 0.060), while HSP-70 (COOL, 1.9-fold, p = 0.207; HOT, 2.4-fold, p = 0.045), PGC-1α (COOL, 3.8-fold, p = 0.424; HOT, 8.4-fold, p = 0.010), HIF-1α (COOL, 1.6-fold, p = 0.315; HOT, 2.2-fold, p = 0.018) and PDK4 (COOL, 7.6-fold, p = 0.412; HOT, 14.1-fold, p = 0.047) mRNA increased significantly only in HOT at 4 h after exercise. These data indicate that acute exercise in a hot environment facilitates protective response to heat stress (HSP-70), mitochondrial biogenesis (PGC-1α and HIF-1α) and fatty acid oxidation (PDK4).

The effects of body mass reduction on the anaerobic power and selected somatic characteristics of Greco-Roman wrestlers

Study aim: The aim of the study was to determine the effects of body weight loss on the levels of somatic features and peak power of senior Greco-Roman wrestlers from the Polish national team.

Material and methods: The study included 14 males, i.e. 7 wrestlers reducing their body weight and 7 wrestlers not changing their body weight. Seven days prior to a competition, the study participants began a 5-day process of losing body weight. The process involved reducing caloric intake from 3460 ± 683.1 kcal to 2160.0 ± 423.5 kcal per day.

Results: Over the period of 5 days, the participants reduced their body weight by 3.1 ± 0.7 kg (p < 0.001). There occurred a decrease in the value of endomorphy (p < 0.01) and mesomorphy (p < 0.05) and an increase in the value of ectomorphy (p < 0.001). Peak power did not change (1296 ± 100.9 W on day 5 of the examination), nor did relative peak power (19.9 ± 1.1W/kg).

Conclusions: In wrestlers 5-day rapid weight loss protocol may result in decrease of body circumferences and consecutive changes in somatic type without concurrent loss in lower-limb peak power.

Corticospinal and peripheral responses to heat-induced hypo-hydration: potential physiological mechanisms and implications for neuromuscular function

Heat-induced hypo-hydration (hyperosmotic hypovolemia) can reduce prolonged skeletal muscle performance; however, the mechanisms are less well understood and the reported effects on all aspects of neuromuscular function and brief maximal contractions are inconsistent. Historically, a 4–6% reduction of body mass has not been considered to impair muscle function in humans, as determined by muscle torque, membrane excitability and peak power production. With the development of magnetic resonance imaging and neurophysiological techniques, such as electromyography, peripheral nerve, and transcranial magnetic stimulation (TMS), the integrity of the brain-to-muscle pathway can be further investigated. The findings of this review demonstrate that heat-induced hypo-hydration impairs neuromuscular function, particularly during repeated and sustained contractions. Additionally, the mechanisms are separate to those of hyperthermia-induced fatigue and are likely a result of modulations to corticospinal inhibition, increased fibre conduction velocity, pain perception and impaired contractile function. This review also sheds light on the view that hypo-hydration has ‘no effect’ on neuromuscular function during brief maximal voluntary contractions. It is hypothesised that irrespective of unchanged force, compensatory reductions in cortical inhibition are likely to occur, in the attempt of achieving adequate force production. Studies using single-pulse TMS have shown that hypo-hydration can reduce maximal isometric and eccentric force, despite a reduction in cortical inhibition, but the cause of this is currently unclear. Future work should investigate the intracortical inhibitory and excitatory pathways within the brain, to elucidate the role of the central nervous system in force output, following heat-induced hypo-hydration.

Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies

A rise in body core temperature and loss of body water via sweating are natural consequences of prolonged exercise in the heat. This review provides a comprehensive and integrative overview of how the human body responds to exercise under heat stress and the countermeasures that can be adopted to enhance aerobic performance under such environmental conditions. The fundamental concepts and physiological processes associated with thermoregulation and fluid balance are initially described, followed by a summary of methods to determine thermal strain and hydration status. An outline is provided on how exercise-heat stress disrupts these homeostatic processes, leading to hyperthermia, hypohydration, sodium disturbances, and in some cases exertional heat illness. The impact of heat stress on human performance is also examined, including the underlying physiological mechanisms that mediate the impairment of exercise performance. Similarly, the influence of hydration status on performance in the heat and how systemic and peripheral hemodynamic adjustments contribute to fatigue development is elucidated. This review also discusses strategies to mitigate the effects of hyperthermia and hypohydration on exercise performance in the heat by examining the benefits of heat acclimation, cooling strategies, and hyperhydration. Finally, contemporary controversies are summarized and future research directions are provided.

Individual fluid plans versus ad libitum on hydration status in minor professional ice hockey players

Background

Despite exercising in cool environments, ice hockey players exhibit several dehydration risk factors. Individualized fluid plans (IFPs) are designed to mitigate dehydration by matching an individual’s sweat loss in order to optimize physiological systems and performance.

Methods

A randomized control trial was used to examine IFP versus ad libitum fluid ingestion on hydration in 11 male minor professional ice hockey players (mean age = 24.4 ± 2.6 years, height = 183.0 ± 4.6 cm, weight = 92.9 ± 7.8 kg). Following baseline measures over 2 practices, participants were randomly assigned to either control (CON) or intervention (INT) for 10 additional practices. CON participants were provided water and/or carbohydrate electrolyte beverage to drink ad libitum. INT participants were instructed to consume water and an electrolyte-enhanced carbohydrate electrolyte beverage to match sweat and sodium losses. Urine specific gravity, urine color, and percent body mass change characterized hydration status. Total fluid consumed during practice was assessed.

Results

INT consumed significantly more fluid than CON (1180.8 ± 579.0 ml vs. 788.6 ± 399.7 ml, p = 0.002). However, CON participants replaced only 25.4 ± 12.9% of their fluid needs and INT 35.8 ± 17.5%. Mean percent body mass loss was not significantly different between groups and overall indicated minimal dehydration (<1.2% loss). Pre-practice urine specific gravity indicated CON and INT began hypohydrated (mean = 1.024 ± 0.007 and 1.024 ± 0.006, respectively) and experienced dehydration during practice (post = 1.026 ± 0.006 and 1.027 ± 0.005, respectively, p < 0.001). Urine color increased pre- to post-practice for CON (5 ± 2 to 6 ± 1, p < 0.001) and INT (5 ± 1 to 6 ± 1, p < 0.001).

Conclusions

Participants consistently reported to practice hypohydrated. Ad libitum fluid intake was not significantly different than IFP on hydration status. Based on urine measures, both methods were unsuccessful in preventing dehydration during practice, suggesting practice-only hydration is inadequate to maintain euhydration in this population when beginning hypohydrated.

The Effect of Fluid Intake Following Dehydration on Subsequent Athletic and Cognitive Performance: a Systematic Review and Meta-analysis

Background

The deleterious effects of dehydration on athletic and cognitive performance have been well documented. As such, dehydrated individuals are advised to consume fluid in volumes equivalent to 1.25 to 1.5 L kg⁻¹ body mass (BM) lost to restore body water content. However, individuals undertaking subsequent activity may have limited time to consume fluid. Within this context, the impact of fluid intake practices is unclear. This systematic review investigated the effect of fluid consumption following a period of dehydration on subsequent athletic and cognitive performance.

Methods

PubMed (MEDLINE), Web of Science (via Thomas Reuters) and Scopus databases were searched for articles reporting on athletic (categorized as: continuous, intermittent, resistance, sport-specific and balance exercise) or cognitive performance following dehydration of participants under control (no fluid) and intervention (fluid intake) conditions. Meta-analytic procedures determined intervention efficacy for continuous exercise performance.

Results

Sixty-four trials (n = 643 participants) derived from 42 publications were reviewed. Dehydration decreased BM by 1.3–4.2%, and fluid intake was equivalent to 0.4–1.55 L kg⁻¹ BM lost. Fluid intake significantly improved continuous exercise performance (22 trials), Hedges’ g = 0.46, 95% CI 0.32, 0.61. Improvement was greatest when exercise was performed in hotter environments and over longer durations. The volume or timing of fluid consumption did not influence the magnitude of this effect. Evidence indicating a benefit of fluid intake on intermittent (10 trials), resistance (9 trials), sport-specific (6 trials) and balance (2 trials) exercise and on cognitive performance (15 trials) was less apparent and requires further elucidation.

Conclusions

Fluid consumption following dehydration may improve continuous exercise performance under heat stress conditions, even when the body water deficit is modest and fluid intake is inadequate for complete rehydration.

Electronic supplementary material

The online version of this article (doi:10.1186/s40798-017-0079-y) contains supplementary material, which is available to authorized users.

Influence of Dehydration on Intermittent Sprint Performance

This study examined the effects of dehydration on intermittent sprint performance and perceptual responses. Eight male collegiate baseball players completed intermittent sprints either dehydrated (DEHY) by 3% body mass or euhydrated (EU). Body mass was reduced through exercise in the heat with controlled fluid restriction occurring 1 day before the trial. Participants completed twenty-four 30-m sprints divided into 3 bouts of 8 sprints with 45 seconds of rest between each sprint and 3 minutes between each bout. Perceived recovery status (PRS) scale was recorded before the start of each trial. Heart rate (HR), ratings of perceived exertion (RPE) (0-10 OMNI scale), and perceived readiness (PR) scale were recorded after every sprint, and session RPE (SRPE) was recorded 20 minutes after completing the entire session. A 2 (condition) × 3 (bout of sprints) repeated-measures ANOVA revealed a significant main effect of condition on mean sprint time (p = 0.03), HR (p < 0.01), RPE (p = 0.01), and PR (p = 0.02). Post hoc tests showed significantly faster mean sprint times for EU vs. DEHY during the second (4.87 ± 0.29 vs. 5.03 ± 0.33 seconds; p = 0.01) and third bouts of sprints (4.91 ± 0.29 vs. 5.12 ± 0.44 seconds; p = 0.02). Heart rate was also significantly lower (p ≤ 0.05) for EU during the second and third bouts. Post hoc measures also showed significantly impaired (p ≤ 0.05) feelings of recovery (PRS) before exercise and increased (p ≤ 0.05) perceptual strain before each bout (PR) during the second and third bouts of repeated sprint work (i.e., RPE and PR) and after the total session (SRPE) in the DEHY condition. Dehydration impaired sprint performance, negatively altered perception of recovery status before exercise, and increased RPE and HR response.

Maintaining hydration with a carbohydrate–electrolyte solution improves performance, thermoregulation, and fatigue during an ice hockey scrimmage

Research in “stop-and-go” sports has demonstrated that carbohydrate ingestion improves performance and fatigue, and that dehydration of ∼1.5%–2% body mass (BM) loss results in decreased performance, increased fatigue, and increased core temperature. The purpose of this investigation was to assess the physiological, performance, and fatigue-related effects of maintaining hydration with a carbohydrate–electrolyte solution (CES) versus dehydrating by ∼2% BM (no fluid; NF) during a 70-min ice hockey scrimmage. Skilled male hockey players (n = 14; age, 21.3 ± 0.2 years; BM, 80.1 ± 2.5 kg; height, 182.0 ± 1.2 cm) volunteered for the study. Subjects lost 1.94% ± 0.1% BM in NF, and 0.12% ± 0.1% BM in CES. Core temperature (Tc) throughout the scrimmage (10–50 min) and peak Tc (CES: 38.69 ± 0.10 vs. NF: 38.92 ± 0.11 °C; p < 0.05) were significantly reduced in CES compared with NF. Players in CES had increased mean skating speed and time at high effort between 30–50 min of the scrimmage. They also committed fewer puck turnovers and completed a higher percentage of passes in the last 20 min of play compared with NF. Postscrimmage shuttle skating performance was improved in CES versus NF and fatigue was lower following the CES trial. The results indicated that ingesting a CES to maintain BM throughout a 70-min hockey scrimmage resulted in improved hockey performance and thermoregulation, and decreased fatigue as compared with drinking no fluid and dehydrating by ∼2%.

Physiological responses of medical team members to a simulated emergency in tropical field conditions

INTRODUCTION

Responses to physical activity while wearing personal protective equipment in hot laboratory conditions are well documented. However less is known of medical professionals responding to an emergency in hot field conditions in standard attire. Therefore, the purpose of this study was to assess the physiological responses of medical responders to a simulated field emergency in tropical conditions.

METHODS

Ten subjects, all of whom were chronically heat-acclimatized health care workers, volunteered to participate in this investigation. Participants were the medical response team of a simulated field emergency conducted at the Northern Territory Emergency Services training grounds, Yarrawonga, NT, Australia. The exercise consisted of setting up a field hospital, transporting patients by stretcher to the hospital, triaging and treating the patients while dressed in standard medical response uniforms in field conditions (mean ambient temperature of 29.3°C and relative humidity of 50.3%, apparent temperature of 27.9°C) for a duration of 150 minutes. Gastrointestinal temperature was transmitted from an ingestible sensor and used as the index of core temperature. An integrated physiological monitoring device worn by each participant measured and logged heart rate, chest temperature and gastrointestinal temperature throughout the exercise. Hydration status was assessed by monitoring the change between pre- and post-exercise body mass and urine specific gravity (USG).

RESULTS

Mean core body temperature rose from 37.5°C at the commencement of the exercise to peak at 37.8°C after 75 minutes. The individual peak core body temperature was 38.5°C, with three subjects exceeding 38.0°C. Subjects sweated 0.54 L per hour and consumed 0.36 L of fluid per hour, resulting in overall dehydration of 0.7% of body mass at the cessation of exercise. Physiological strain index was indicative of little to low strain.

CONCLUSIONS

The combination of the unseasonably mild environmental conditions and moderate work rates resulted in minimal heat storage during the simulated exercise. As a result, low sweat rates manifested in minimal dehydration. When provided with access to fluids in mild environmental conditions, chronically heat-acclimatized medical responders can meet their hydration requirements through ad libitum fluid consumption. Whether such an observation is replicated under a harsher thermal load remains to be investigated.

The influence of hydration on anaerobic performance: a review

This review examines the influence of dehydration on muscular strength and endurance and on single and repeated anaerobic sprint bouts. Describing hydration effects on anaerobic performance is difficult because various exercise modes are dominated by anaerobic energy pathways, but still contain inherent physiological differences. The critical level of water deficit (approximately 3-4%; mode dependent) affecting anaerobic performance is larger than the deficit (approximately 2%) impairing endurance performance. A critical performance-duration component (> 30 s) may also exist. Moderate dehydration (approximately 3% body weight; precise threshold depends on work/recovery ratio) impairs repeated anaerobic bouts, which place an increased demand on aerobic metabolism. Interactions between dehydration level, dehydration mode, testing mode, performance duration, and work/recovery ratio during repeated bouts make the dehydration threshold influencing anaerobic performance mode dependent.

Effects of heat exposure and 3% dehydration achieved via hot water immersion on repeated cycle sprint performance

This study examined effects of heat exposure with and without dehydration on repeated anaerobic cycling. Males (n = 10) completed 3 trials: control (CT), water-bath heat exposure (∼39°C) to 3% dehydration (with fluid replacement) (HE), and similar heat exposure to 3% dehydration (DEHY). Hematocrit increased significantly from pre to postheat immersion in both HE and DEHY. Participants performed 6 × 15s cycle sprints (30s active recovery). Mean Power (MP) was significantly lower vs. CT (596 ± 66 W) for DEHY (569 ± 72 W), and the difference approached significance for HE (582 ± 76 W, p = 0.07). Peak Power (PP) was significantly lower vs. CT (900 ± 117 W) for HE (870 ± 128 W) and approached significance for DEHY (857 ± 145 W, p = 0.07). Postsprint ratings of perceived exertion was higher during DEHY (6.4 ± 2.0) and HE (6.3 ± 1.6) than CT (5.7 ± 2.1). Combined heat and dehydration impaired MP and PP (decrements greatest in later bouts) with HE performance intermediate to CT and DEHY.

The effects of fluid ingestion on free-paced intermittent-sprint performance and pacing strategies in the heat

The purpose of this study was to examine the effects of fluid ingestion on pacing strategies and performance during intermittent-sprint exercise in the heat. Nine male rugby players performed a habituation session and 2 x 50-min intermittent-sprint protocols at a temperature of 31 degrees C, either with or without fluid. Participants were informed of a third session (not performed) to ensure that they remained blind to all respective conditions. The protocol consisted of a 15-m sprint every minute separated by self-paced bouts of hard running, jogging, and walking for the remainder of the minute. Sprint time, distance covered during self-paced exercise, and vertical jump height before and after exercise were recorded. Heart rate, core temperature, nude mass, capillary blood haematocrit, pH, lactate concentration, perceptual ratings of perceived exertion, thermal stress, and thirst were also recorded. Sprint times (fluid vs. no-fluid: 2.82 +/- 0.11 vs. 2.82 +/- 0.14) and distance covered during self-paced exercise (fluid vs. no-fluid: 4168 +/- 419 vs. 3981 +/- 263 m) were not different between conditions (P = 0.10-0.98) but were progressively reduced to a greater extent in the no-fluid trial (7 +/- 13%) (d = 0.56-0.58). There were no differences (P = 0.22-1.00; d = <0.20-0.84) between conditions in any physiological measures. Perceptual ratings of perceived exertion and thermal stress did not differ between conditions (P = 0.34-0.91; d < or =0.20-0.48). Rating of thirst after exercise was lower in the fluid trial (P = 0.02; d = 0.62-0.73). The present results suggest that fluid availability did not improve intermittent-sprint performance, however did affect pacing strategies with a greater reduction in distance covered of self-paced exercise during the no-fluid trial.

Core temperature responses and match running performance during intermittent-sprint exercise competition in warm conditions

This study investigated the thermoregulatory responses and match running performance of elite team sport competitors (Australian Rules football) during preseason games in a warm environment. During 2 games in dry bulb temperatures above 29 degrees C (>27 degrees C wet bulb globe temperature), 10 players were monitored for core temperature (Tcore) via a telemetric capsule, in-game motion patterns, blood lactate ([La]), body mass changes, urine specific gravity, and pre- and postgame vertical jump performance. The results showed that peak Tcore was achieved during the final quarter at 39.3 +/- 0.7 degrees C and that several players reached values near 40.0 degrees C. Further, the largest proportion of the total rise in Tcore (2.1 +/- 0.7 degrees C) occurred during the first quarter of the match, with only small increases during the remainder of the game. The game distance covered was 9.4 +/- 1.5 km, of which 2.7 +/- 0.9 km was at high-intensity speeds (>14.4 km x h(-1)). The rise in Tcore was correlated with first-quarter high-intensity running velocity (r = 0.72) and moderate-intensity velocity (r = 0.68), second-quarter Tcore and low-intensity activity velocity (r = -0.90), second-quarter Tcore and moderate-intensity velocity (r = 0.88), fourth-quarter rise in Tcore and very-high-intensity running distance (r = 0.70), and fourth-quarter Tcore and moderate-intensity velocity (r = 0.73). Additional results included mean game [La-] values of 8.7 +/- 0.1 mmol x L(-1), change in body mass of 2.1 +/- 0.8 kg, and no change (p > 0.05) in pre- to postgame vertical jump. These findings indicate that the plateau in Tcore may be regulated by the reduction in low-intensity activity and that pacing strategies may be employed during competitive team sports in the heat to ensure control of the internal heat load.

Influence of hypohydration on intermittent sprint performance in the heat

PURPOSE

To examine the effect of hypohydration on physiological strain and intermittent sprint exercise performance in the heat (35.5 +/- 0.6 degrees C, 48.7 +/- 3.4% relative humidity).

METHODS

Eight unacclimatized males (age 23.4 +/- 6.2 y, height 1.78 +/- 0.04 m, mass 76.8 +/- 7.7 kg) undertook three trials, each over two days. On day 1, subjects performed 90 min of exercise/heat-induced dehydration on a cycle ergometer, before following one of three rehydration strategies. On day 2, subjects completed a 36-min cycling intermittent sprint test (IST) with a -0.62 +/- 0.74% (euhydrated, EUH), -1.81 (0.99)% (hypohydrated1, HYPO1), or -3.88 +/- 0.89% (hypohydrated2, HYPO2) body mass deficit.

RESULTS

No difference was observed in average total work (EUH, 3790 +/- 556 kJ; HYPO1, 3785 +/- 628 kJ; HYPO2, 3647 +/- 339 kJ, P = 0.418), or average peak power (EUH, 1315 +/- 129 W; HYPO1, 1304 +/- 175 W; HYPO2, 1282 +/- 128 W, P = 0.356) between conditions on day 2. Total work and peak power output in the sprint immediately following an intense repeated sprint bout during the IST were lower in the HYPO2 condition. Physiological strain index was greater in the HYPO2 vs. the EUH condition, but without changes in metabolic markers.

CONCLUSION

A greater physiological strain was observed with the greatest degree of hypohydration; however, sprint performance only diminished in the most hypohydrated state near the end of the IST, following an intense bout of repeating sprinting.

Effect of recovery intensity on peak power output and the development of heat strain during intermittent sprint exercise while under heat stress

This study compared two intensities of active recovery on intermittent sprint exercise performance and the development of heat strain in hot, humid conditions. Eight male game players completed four Cycling Intermittent Sprint Protocols (CISP) consisting of twenty 2-min periods, each including 10-s passive rest, 5-s maximal sprint against a resistance of 7.5% body mass and 105-s active recovery. The CISP was performed in mean (S.D.) temperate conditions with active recovery intensities of 50% V(O)(2peak) (TEMP50) and 35% V(O)(2peak)(TEMP35) and in hot, humid [35.2 (0.4) degrees C, 80.4 (2.1)% RH] conditions with the same intensities (HOT50 and HOT35, respectively) in a randomised, counterbalanced order. Heat strain (physiological strain index (PSI)) was calculated from rectal temperature and heart rate. All subjects completed the CISP (20 sprints) in TEMP50 and TEMP35. The mean number of sprints completed for HOT50 and HOT35 was 13 (3) and 17 (2), respectively; both of which were lower than TEMP50 and TEMP35 (P<0.01) and different between hot conditions. Reductions in peak power output (PPO) occurred in the TEMP50 and HOT50 by sprint 8 (P<0.05), but in HOT35 a reduction was delayed until sprint 13 (P<0.05). The rate of PSI increase was faster in HOT50 than TEMP50 and HOT35, but peak PSI was not different. By lowering the recovery intensity, one component of the PSI (heart rate) was reduced and intermittent sprint exercise performance was maintained for longer in the heat.

Anaerobic performance when rehydrating with water or commercially available sports drinks during prolonged exercise in the heat

The effects that rehydrating drinks ingested during exercise may have on anaerobic exercise performance are unclear. This study aimed to determine which of four commercial rehydrating drinks better maintains leg power and force during prolonged cycling in the heat. Seven endurance-trained and heat-acclimatized cyclists pedaled for 120 min at 63% maximum oxygen consumption in a hot, dry environment (36 °C; 29% humidity, 1.9 m·s–1 airflow). In five randomized trials, during exercise, subjects drank 2.4 ± 0.1 L of (i) mineral water (WAT; San Benedetto®), (ii) 6% carbohydrate–electrolyte solution (Gatorade® lemon), (iii) 8% carbohydrate–electrolyte solution (Powerade® Citrus Charge), (iv) 8% carbohydrate–electrolyte solution with lower sodium concentration than other sports drinks (Aquarius® orange), or (v) did not ingest any fluid (DEH). Fluid balance, rectal temperature (Trec), maximal cycling power (Pmax), and leg maximal voluntary isometric contraction (MVC) were measured. During DEH, subjects lost 3.7 ± 0.2% of initial body mass, whereas subjects lost only 0.8% ± 0.1% in the other trials (p < 0.05). Final Trec was higher in DEH than in the rest of the trials (39.4 ± 0.1 °C vs. 38.7 ± 0.1 °C; p < 0.05). Pmax was similar among all trials. Gatorade® and Powerade® preserved MVC better than DEH (–3.1% ± 2% and –3.8% ± 2% vs. –11% ± 2%, p < 0.05), respectively, whereas WAT and Aquarius® did not (–6% ± 2%). Compared with DEH, rehydration with commercially available sports drinks during prolonged exercise in the heat preserves leg force, whereas rehydrating with water does not. However, low sodium concentration in a sports drink seems to preclude its ergogenic effects on force.

Exercise-induced homeostatic perturbations provoked by singles tennis match play with reference to development of fatigue

This review addresses metabolic, neural, mechanical and thermal alterations during tennis match play with special focus on associations with fatigue. Several studies have provided a link between fatigue and the impairment of tennis skills proficiency. A tennis player's ability to maintain skilled on-court performance and/or optimal muscle function during a demanding match can be compromised as a result of several homeostatic perturbations, for example hypoglycaemia, muscle damage and hyperthermia. Accordingly, an important physiological requirement to succeed at competitive level might be the player's ability to resist fatigue. However, research evidence on this topic is limited and it is unclear to what extent players experience fatigue during high-level tennis match play and what the physiological mechanisms are that are likely to contribute to the deterioration in performance.

Hydration and physical performance

There is a rich scientific literature regarding hydration status and physical function that began in the late 1800s, although the relationship was likely apparent centuries before that. A decrease in body water from normal levels (often referred to as dehydration or hypohydration) provokes changes in cardiovascular, thermoregulatory, metabolic, and central nervous function that become increasingly greater as dehydration worsens. Similarly, performance impairment often reported with modest dehydration (e.g., -2% body mass) is also exacerbated by greater fluid loss. Dehydration during physical activity in the heat provokes greater performance decrements than similar activity in cooler conditions, a difference thought to be due, at least in part, to greater cardiovascular and thermoregulatory strain associated with heat exposure. There is little doubt that performance during prolonged, continuous exercise in the heat is impaired by levels of dehydration >or= -2% body mass, and there is some evidence that lower levels of dehydration can also impair performance even during relatively short-duration, intermittent exercise. Although additional research is needed to more fully understand low-level dehydration's effects on physical performance, one can generalize that when performance is at stake, it is better to be well-hydrated than dehydrated. This generalization holds true in the occupational, military, and sports settings.

Effects of active warm up on thermoregulation and intermittent-sprint performance in hot conditions

This study examined the effects of active warm up on thermoregulatory responses and intermittent-sprint cycle performance in hot conditions (35.5+/-0.6 degrees C, RH 48.7+/-3.4%). Eight trained males performed a 36-min, intermittent-sprint test (IST) after no (WUP 0), 10-min (WUP 10) or 20-min warm up (WUP 20). The IST contained 2-min blocks consisting of a 4-s sprint, 100s active recovery and 20s passive rest. Twice during the IST, there was a repeated-sprint bout (RSB) comprising five, 2-s sprints separated by approximately 20s. There were no significant differences between trials for mean work (3870+/-757 versus 4028+/-562 versus 3804+/-494Jsprint(-1)), peak power (W) or work decrement (%). However, mean work was significantly less in RSB2 than RSB1 for WUP 20 only (P<0.05). Plasma lactate was significantly higher after active warm up (WUP 20=WUP 10>WUP 0; P<0.05), but not significantly different between conditions following either RSB. Rectal temperature (T(re)) was significantly higher after active warm up (37.0+/-0.3 versus 37.3+/-0.3 versus 37.7+/-0.1 degrees C for WUP0, WUP10 and WUP20, respectively) and throughout the IST. The longer active warm up resulted in a greater increase in T(re) and was associated with a decrease in short-term repeated-sprint ability (with incomplete recovery), but not prolonged, intermittent-sprint performance in the heat. As active warm up did not improve performance (<40min), team-sport athletes may minimise changes in T(re) (and the likelihood of heat illness) by avoiding excessive warm up when competing in the heat.

Progressive dehydration causes a progressive decline in basketball skill performance

PURPOSE

To determine the effect of 1, 2, 3, and 4% dehydration (DEH) versus euhydration (EUH) on basketball performance in adult male players.

METHODS

Seventeen 17- to 28-yr-old male basketball players completed 3 h of interval treadmill walking (40 degrees C and 20% relative humidity) with or without fluid replacement. Subjects completed six trials in random order: 1) EUH with a carbohydrate-electrolyte solution (CES), 2) EUH control (flavored water with 0% carbohydrate and 18 mM sodium), 3) 1% DEH, 4) 2% DEH, 5) 3% DEH, and 6) 4% DEH. After a 70-min recovery period, subjects performed a sequence of continuous basketball drills designed to simulate a fast-paced game. Measures of overall skill performance during the 80-min game included 1) total time to complete basketball-specific movement drills (sprinting, defensive slides, sprinting-defensive slides combination, and repetitive jumping drills) and 2) total number of shots (foul-line and baseline jump shots, layups, three-point, 15-ft, free throws) made per game.

RESULTS

Performance during all timed and shooting drills declined progressively as % DEH increased. Total time to complete basketball-specific movement drills was slower (1%: + 7 +/- 6; 2%: + 20 +/- 5 (P < 0.05); 3%: + 26 +/- 7 (P < 0.005); 4%: + 57 +/- 9 (P < 0.0001) s), and fewer shots were made during DEH versus EUH control (1%: -5 +/- 1; 2%: -6 +/- 2 (P < 0.05); 3%: -8 +/- 2 (P < 0.005); 4%: -10 +/- 1 (P < 0.0001) shots made). There were no significant differences in performance between CES and EUH control.

CONCLUSION

Basketball players experienced a progressive deterioration in performance as DEH progressed from 1 to 4%. The threshold, or % DEH at which the performance decrement reached statistical significance, was 2% for combined timed and shooting drills.

Effects of pre-cooling procedures on intermittent-sprint exercise performance in warm conditions

The aim of this study was to determine whether pre-cooling procedures improve both maximal sprint and sub-maximal work during intermittent-sprint exercise. Nine male rugby players performed a familiarisation session and three testing sessions of a 2 x 30-min intermittent sprint protocol, which consisted of a 15-m sprint every min separated by free-paced hard-running, jogging and walking in 32 degrees C and 30% humidity. The three sessions included a control condition, Ice-vest condition and Ice-bath/Ice-vest condition, with respective cooling interventions imposed for 15-min pre-exercise and 10-min at half-time. Performance measures of sprint time and % decline and distance covered during sub-maximal exercise were recorded, while physiological measures of core temperature (T (core)), mean skin temperature (T (skin)), heart rate, heat storage, nude mass, rate of perceived exertion, rate of thermal comfort and capillary blood measures of lactate [La(-)], pH, Sodium (Na(+)) and Potassium (K(+)) were recorded. Results for exercise performance indicated no significant differences between conditions for the time or % decline in 15-m sprint efforts or the distance covered during sub-maximal work bouts; however, large effect size data indicated a greater distance covered during hard running following Ice-bath cooling. Further, lowered T (core), T (skin), heart rate, sweat loss and thermal comfort following Ice-bath cooling than Ice-vest or Control conditions were present, with no differences present in capillary blood measures of [La(-)], pH, K(+) or Na(+). As such, the ergogenic benefits of effective pre-cooling procedures in warm conditions for team-sports may be predominantly evident during sub-maximal bouts of exercise.

Effects of Active versus Passive Recovery on Thermoregulatory Strain and Performance in Intermittent-Sprint Exercise

PURPOSE

Team sports contain high-intensity sprints separated by active recovery (AR) and passive recovery (PR). The beneficial effects of AR on repeated-sprint performance, for short exercise duration, in thermoneutral environments, are well known. However, team sports are often performed in hot environments for prolonged periods. Therefore, the aim was to investigate the thermal strain of AR versus PR during prolonged, intermittent-sprint exercise.

METHODS

Eight men performed two intermittent-sprint tests in the heat (35 degrees C, 44% relative humidity (RH)), with either AR or PR.

RESULTS

No differences were found between conditions for mean work (AR: 3739.5 +/- 204.7 J; PR: 3814.0 +/- 161.3 J) or power per sprint (AR: 1257 +/- 64 W; PR: 1245 +/- 47 W). AR was associated with a significantly higher heart rate (HR), muscle (Tmu), rectal temperature (Tre), body temperature (Tb), and skin temperature (Tsk) after 7, 10, and 25 min, respectively. Body heat storage, and physiological and cumulative heat-strain indices, were significantly higher in AR compared with PR. The differences in Tmu and thermoregulatory strain between AR and PR were greater than the differences in Tre and Tb.

CONCLUSIONS

These results likely can be attributed to a greater rate of whole-body heat loss during the AR protocol. Because AR has previously been associated with a greater muscle pump, a greater blood flow to surface veins and inactive musculature may have been maintained, allowing greater heat dissipation than during PR, when blood was likely to be pooling in the legs. Despite the greater increase in body temperature and heat strain in AR than in PR, there was no difference in performance, possibly because critical temperature levels were not reached in this study.

Two percent dehydration impairs and six percent carbohydrate drink improves boys basketball skills

PURPOSE

To determine the effects of exercise heat-induced two percent dehydration (DEH) and euhydration (EUH) with a six percent carbohydrate-electrolyte solution (CES) compared with placebo EUH (P EUH) on basketball skills in skilled young players.

METHODS

Fifteen 12- to 15-yr-old boys underwent three separate 2-h exercise heat exposures (double blind, random order): 2% DEH by limiting fluid intake during exercise in the heat and basketball skill drills, EUH (no net weight change) with a 6% CES, and EUH with a flavored water placebo (P EUH). After recovery, subjects performed an orchestrated sequence of continuous basketball drills designed to simulate a game (12-min quarters + a 10-min halftime). Performance measures and component drills inherent to basketball included various individual and combined shooting percentages (3-point, 15-foot, free-throw shots), sprint (suicides, court widths), lateral movement (zigzags, lane slides), and defensive drill (combining lateral and front-to-back movement) times.

RESULTS

Compared with P EUH (53 +/- 11%), combined shooting percentage was impaired by 2% DEH (45 +/- 9%; P = 0.002) and improved by CES intake (60 +/- 8%; P = 0.003). Total sprint times showed a similar effect (83 +/- 10 vs 78 +/- 9 vs 76 +/- 9 s; DEH vs P EUH vs CES; P < 0.001 and P = 0.04, respectively). Total lateral movement times were impaired by 2% DEH (73 +/- 8 vs 68 +/- 8 s; P = 0.001). CES improved total defensive drill times compared with 2% DEH (77 +/- 10 vs 82 +/- 10; P = 0.006).

CONCLUSION

Deterioration in basketball skill performance accompanies two percent dehydration in skilled 12- to 15-yr-old basketball players. Additionally, EUH with a 6% CES significantly improves shooting performance and on-court sprinting over EUH with water.

No effect of moderate hypohydration or hyperthermia on anaerobic exercise performance

PURPOSE

This study examined the effects of hypohydration and moderate hyperthermia (core temperature elevation) on anaerobic exercise performance in a temperate environment.

METHODS

Eight active males completed two passive heat exposure trials (180 min, 45 degrees C, 50% rh) with (EUH) and without (HYP) fluid replacement. A single 15-s Wingate anaerobic test (WAnT) was used to assess anaerobic performance (peak power, mean power, and fatigue index) before (-180 min) and again at three time points after passive heat exposure to include immediately (0 min), 30 min, and 60 min after in a temperate environment (22 degrees C). Rectal temperature (Tc) was measured throughout the experiment.

RESULTS

HYP reduced body mass (2.7+/-0.7%) (P<0.05) but had no effect on any WAnT performance measure. Passive heat exposure elicited moderate hyperthermia in both trials (EUH: 0.6 degrees C; HYP: 1.0 degrees C) and returned to baseline within 30-60 min following similar decay curves. HYP Tc remained higher (0.4 degrees C) than EUH throughout testing (P<0.05), but moderate hyperthermia itself produced no independent effect on anaerobic exercise performance in either trial.

CONCLUSIONS

This study demonstrates that neither moderate HYP nor the moderate hyperthermia accompanying HYP by passive heat exposure affect anaerobic exercise performance in a temperate environment.

Nutritional strategies for football: Counteracting heat, cold, high altitude, and jet lag

Environmental factors often influence the physical and mental performance of football players. Heat, cold, high altitude, and travel across time zones (i.e. leading to jet lag) act as stressors that alter normal physiological function, homeostasis, metabolism, and whole-body nutrient balance. Rather than accepting performance decrements as inevitable, well-informed coaches and players should plan strategies for training and competition that offset environmental challenges. Considering the strength of scientific evidence, this paper reviews recommendations regarding nutritional interventions that purportedly counterbalance dehydration, hyperthermia, hypothermia, hypoxia, acute or chronic substrate deficiencies, sleep loss, and desynchronization of internal biological clocks.

Precooling leg muscle improves intermittent sprint exercise performance in hot, humid conditions

We used three techniques of precooling to test the hypothesis that heat strain would be alleviated, muscle temperature (Tmu) would be reduced, and as a result there would be delayed decrements in peak power output (PPO) during exercise in hot, humid conditions. Twelve male team-sport players completed four cycling intermittent sprint protocols (CISP). Each CISP consisted of twenty 2-min periods, each including 10 s of passive rest, 5 s of maximal sprint against a resistance of 7.5% body mass, and 105 s of active recovery. The CISP, preceded by 20 min of no cooling (Control), precooling via an ice vest (Vest), cold water immersion (Water), and ice packs covering the upper legs (Packs), was performed in hot, humid conditions (mean +/- SE; 33.7 +/- 0.3 degrees C, 51.6 +/- 2.2% relative humidity) in a randomized order. The rate of heat strain increase during the CISP was faster in Control than Water and Packs (P < 0.01), but it was similar to Vest. Packs and Water blunted the rise of Tmu until minute 16 and for the duration of the CISP (40 min), respectively (P < 0.01). Reductions in PPO occurred from minute 32 onward in Control, and an increase in PPO by approximately 4% due to Packs was observed (main effect; P < 0.05). The method of precooling determined the extent to which heat strain was reduced during intermittent sprint cycling, with leg precooling offering the greater ergogenic effect on PPO than either upper body or whole body cooling.

Influence of Diuretic-Induced Dehydration on Competitive Sprint and Power Performance

Diuretic-induced dehydration impairs prolonged running performance (> 1500 m). Sprinting performance may suffer by similar mechanisms (i.e., altered cardiovascular strain, heat storage, and metabolism) or may improve because of reduced mass to accelerate and carry.

PURPOSE

To examine sprint and power performance after diuretic-induced dehydration.

METHODS

After six sprint practice sessions, nine male former sprinters (mean +/- SD; age, 21 +/- 2 yr; body mass (BM), 80.0 +/- 5.2 kg; height, 1.78 +/- 0.08 m; body fat, 14 +/- 4%) participated in a 50-m race, a 200-m race, a 400-m race, and a vertical jump on an indoor synthetic track, once when dehydrated (40-mg furosemide; DD) and once with no diuretic (CON) using a counter-balanced crossover design. Plasma volume change (%deltaPV), heart rate (HR), blood pressure, rectal temperature, serum electrolytes, plasma lactate, plasma glucose, rating of perceived exertion, thirst, and thermal sensations were measured before and after each race.

RESULTS

Sprint times (DD vs CON) for the 50 m (6.72 +/- 0.28 vs 6.73 +/- 0.29 s), 200 m (25.95 +/- 1.20 vs 26.21 +/- 1.42 s), and 400 m (59.01 +/- 4.26 vs 58.68 +/- 3.68 s) were similar for both conditions, as was vertical jump height (0.67 +/- 0.10 vs 0.66 +/- 0.11 m). This occurred despite losing 2.2 +/- 0.4% BM and 7.3 +/- 6.7%deltaPV (50/200 m) or 2.5 +/- 0.4% BM and 7.1 +/- 2.7% deltaPV (VJ/400 m) in response to DD.

CONCLUSIONS

Diuretic-induced dehydration was not detrimental to sprint and power performance. Metabolic, thermoregulatory, and cardiovascular variables were not significantly altered by DD. Furthermore, the theoretical benefit of dehydration on performance (i.e., BM reduction) was not supported in this subject cohort.

Alterations in energy metabolism during exercise and heat stress

Much of the research that has examined the interaction between metabolism and exercise has been conducted in comfortable ambient conditions. It is clear, however, that environmental temperature, particularly extreme heat, is a major practical issue one must consider when examining muscle energy metabolism. When exercise is conducted in very high ambient temperatures, the gradient for heat dissipation is significantly reduced which results in changes to thermoregulatory mechanisms designed to promote body heat loss. This can ultimately impact upon hormonal and metabolic responses to exercise which act to alter substrate utilisation. In general, the literature examining metabolic responses to exercise and heat stress has demonstrated a shift towards increased carbohydrate use and decreased fat use. Although glucose production appears to be augmented during exercise in the heat, glucose disposal and utilisation appears to be unaltered. In contrast, glycogen use has been consistently demonstrated to be augmented during exercise in the heat. This increase in glycogenolysis is observed via both aerobic and anaerobic pathways. Although several hypotheses have been proposed as mechanisms for the substrate shift towards greater carbohydrate metabolism during exercise and heat stress, recent work suggests that an augmented sympatho-adrenal response and intramuscular temperature may be responsible for such a phenomenon.